Trying to add digits times their index - c++

Question

I'm trying to add the digits times their index in order to create a hash, but this doesn't seem to give the correct output. Can anyone tell me what's wrong here?
int main(){
int i, hash=0, input;
char temp[30];
cin>>input;
itoa (input, temp, 10);
for(i=0; i<(sizeof(temp)/sizeof(*temp)); i++){
hash+=((temp[i])*i);
}
cout<<hash;
return 0;
}

Answer 1

It's because itoa() returns a C-String.
Every C-String is a buffer (just like temp[30]), but not every buffer is a C-String.
The C-String must contain characters from a list of valid things (alphanumerical + some symbols + etc) and MUST finish with the "string terminator" (the character '\0')
Your temp variable is not initialized (we could say it was born as a generic buffer, not a string yet), so after itoa() you will have your "number" converted to string (your buffer just became a C-String), but after the string terminator ('\0') you will have any trash, once you did not initialized it!
Your buffer will be like this:
temp[30] = { '1', '2', '3', '\0', ?, ?, ?, ..., ? }
The ?'s could be anything...
Because you loop over the whole buffer temp[30] you will be making a sum of different things all the time.
Solution 1: Initialize char temp[30] = { 0 } (a little stupid, but valid)
Solution 2: Loop up to the "length" of the C-String temp, instead of the whole buffer (this
one makes more sense!)
This this:
size_t length( strlen( temp ) );
for ( size_t i( 0 ); i < length; i++ )
{
hash += (int)i * temp[ i ];
}

Answer 2

Yes, you are hashing all the 30 bytes of temp[30] instead of hashing only the ascii representation of the integer number you are reading from stdin.
I think this is closer to what you want:
#include <iostream>
using namespace std;
int main() {
int hash = 0;
// Read input from stdin
std::string input;
std::cin >> input;
// Make sure it contains only numbers
if (input.find_first_not_of("0123456789") != std::string::npos) {
std::cout << "Input doesn't contain only digits" << std::endl;
return 1;
}
for (int i = 0; i < input.size(); i++) {
hash += (input[i] - '0') * i;
}
std::cout << hash << std::endl;
return 0;
}

Answer 3

That temp[] buffer is not completely filled by itoa(), so you will be hashing garbage if you loop over i=0 to (sizeof temp)-1. Try:
for (size_t i=0, n=strlen(temp); i<n; ++i)
{
hash += (int)i*temp[i];
}
You'll need <cstring> included to define strlen().
On second thought, the no-<cstring> approach is probably better:
for (int i=0; temp[i] != 0; ++i)
{
hash += i*temp[i];
}
That eliminates the (int) cast too.